This invention relates to polymerization of olefins and particularly relates to Ziegler-Natta or metallocene catalyzed gas-phase polymerization of olefins including alpha-olefins and ethylene in the presence of a radical generating compound.
Ziegler-Natta and metallocene catalyzed olefin polymerization in the gas phase now is well-known in the art. Known gas-phase processes include horizontally and vertically stirred sub-fluidized bed reactor systems, fluidized bed systems, as well as multi-zone circulating reactor systems in which, typically, a fluidized zone is combined with a slower moving reactive zone, such as described in U.S. Pat. No. 6,689,845 incorporated by reference herein. These systems have been used to polymerize ethylene and C3-C10+ alpha olefins as homopolymers or copolymers. Typically, olefin polymers formed in the gas phase are polyethylene, polypropylene, copolymers of ethylene and C3-C10+ olefin monomers, and copolymers of propylene with ethylene and C4+ olefin monomers either as statistical (random) copolymers or multi-phasic (rubber-modified or impact) copolymers.
Post-polymerization reactor modification of olefin polymers with free radical generating species, such as organic peroxides, also is well known in the art. Typically, such modification is performed in an extruder in which formed polymer is melted by mechanical or external heating, extruded through strand dies, and chopped into discrete pellets. Post reactor modification is used in polyethylene to introduce crosslinks among polymer chains to increase molecular weight and modify polymer properties. In propylene polymers, post-reactor extruder modification with peroxides is used to decrease polymer molecular weight through chain scission or cleaving, thereby producing, “controlled rheology” product.
Properties of substantially linear olefin polymers are known to be altered by creating branching of the linear chain or attaching hydrocarbon moieties along the linear chain, although actual formation of such branched linear olefin polymers may be difficult to control, especially in a continuous reactor produced polymer. This invention provides a practical method of producing branched olefin polymers in a continuous gas-phase reactor system.
High melt strength propylene polymers are discussed by Rätzsch et al., Progress in Polymer Science, 27 (2002) 1195-1282, incorporated by reference herein. Post-reactor processing in an extruder with certain peroxides to produce high melt strength propylene polymers is described in U.S. Pat. Nos. 6,103,833, 6,323,289, 6,620,892, and 5,416,169. For this method, extruder conditions are generally set at lower temperatures when compared to normal extrusion processes. The lower extruder temperature typically results in a greater power demand for mixing this material. Therefore, production of high melt strength polymer via this peroxide method will result in higher energy costs, in addition to the added costs for the special peroxide.
In some polyolefins, such as propylene polymers, there is a need in certain uses for a polymer with increased melt strength. For example, higher melt strength is needed for propylene polymers used in thermoforming applications. Current techniques to produce these types of materials employ expensive post reactor processing such as electron beam treatment, as discussed in U.S. Pat. Nos. 5,554,668 and 5,605,936, and processing with special peroxides under controlled conditions as mentioned above. Other approaches to branched propylene polymers use macromonomers as described in U.S. Pat. Nos. 6,184,327 and 6,423,793.
However there is a need for efficient direct reactor formation of branched polyolefins, such as branched propylene polymers, without use of post-reactor peroxide treatment or use of specially prepared macromonomers.